Field
This disclosure relates to power converters and specifically to power converters operating from an AC primary power source.
Description of the Related Art
FIG. 1A is a partial schematic diagram of a 480 volt 3 phase wye power distribution system widely used in industrial and commercial buildings in the United States. T1 is the secondary winding of a 3 phase power distribution transformer. The outputs from T1 are three AC voltages commonly identified as line A (LA), line B (LB), and line C (LC). LA, LB, and LC are separated in phase by 120 degrees. The AC voltage between any two of LA, LB, and LC is nominally 480 volts RMS. The voltage between each one of LA, LB, and LC and a neutral (N) is nominally 277 volts RMS.
Large loads, such as air conditioning compressor motors and elevator motors may be powered by all three voltage phases (not shown). Smaller loads, such as building lighting fixtures, may be connected between any one of LA, LB, and LC and the neutral N. In FIG. 1A resistors RA and RC represent such loads.
FIG. 1B illustrates a failure mode in a 3 phase wye power distribution system in which the neutral connection is broken, either within the power distribution system or the customer premises. In the event of this failure, loads RA and RC do not receive 277 volts as expected. Rather, 480 volts is applied to the series combination of RA and RC. The voltage applied to each of RA and RC depends upon their relative resistance values. For example, if RA is a relatively high power load having low resistance and RC is a low power load having high resistance, the voltage across RC may approach 480 volts.
FIG. 1C illustrates a similar failure mode in a split phase 120 volt/240 volt power distribution system commonly used in residential buildings in the United States. In this case, if the neutral connection is broken, loads RD and RE do not receive 120 volts as expected. Rather, 240 volts is applied to the series combination of RD and RE. The voltage applied to RD and RE depends upon their relative resistance values. For example, if RD is a relatively high power load having low resistance and RE is a low power load having high resistance, the voltage across RE may approach 240 volts.
In an event of either of the AC input overvoltage conditions illustrated in FIG. 1B and FIG. 1C, the failure is likely to be prolonged, rather than transient. Thus equipment intended for operation from one phase of a multi-phase power distribution system may be subjected to an extended AC input overvoltage of more than 170% of the nominal input voltage. Failure or disruption of the operation of such equipment may be permitted so long as the failure occurs without creating a secondary hazardous condition such as a fire or an electrical shock hazard.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.